A double membrane pump of this kind is described in German Offenlegungsschrift 33 10 131. In this double membrane pump the two membranes are connected together by a coupling rod and the pressure fluid chambers are located in the region between the membranes while the product chambers are located outside. The actuating member is arranged parallel to the coupling rod and consists of an axially displaceable rod projecting from the slide valve housing and arranged coaxially in the slide valve. This rod acts in both directions through a compression spring on the slide valve which is held in its end positions by spring loaded ball catches until the force of the springs arranged coaxially on the actuating rod exceeds the retaining force. Driven by the force of the springs, the slide then speeds to the opposite control position and brings about reversal of the movement of the membrane. In this way the valve slide is caused to reciprocate between two stable end positions.
Since the known double membrane pump only has movable suction and pressure valves in the region of the product chambers, and elsewhere only static seals, it is well suited for pumping high purity products such as acids, caustic alkalis and solvents in the semiconductor industry because there is little risk of abraded particles. However the flow path constitutes a disadvantage, since the product being pumped has to pass around the control block with the membranes, the coupling rod and the slide valve, which gives rise to a large surface in contact with the product, and four changes in the direction of flow are necessary between intake and outlet. Moreover a large number of seals are needed. Finally in this arrangement there is also the risk of dimensional changes with variations in temperature. Should this lead to difficulties in the control of the pumping fluid, the whole pump, including the parts in contact with the product, has to be dismantled.
Furthermore, in the known double membrane pump the membrane is sealed by a simple annular bead that has to ensure radial retention as well as for sealing. This is unsatisfactory, since in this region the membrane is highly stressed, and when a flowable membrane material such as PTFE is used the bead does not guarantee perfect retention and sealing in the long term.